1. Field of the Invention
This invention pertains in general to pressurizers for pressurized water nuclear reactor power generating systems and more particularly to the thermal liners attached to the interior of the surge nozzles for such pressurizers.
2. Related Art
The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated and in heat exchange relationship with a secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side.
For the purpose of illustration, FIG. 1 shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel 10 having a closure head 12 enclosing a nuclear core 14. A liquid reactor coolant, such as water, is pumped into the vessel 10 by pump 16 through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18, typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16 completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel 10 by reactor coolant piping 20. The primary side is maintained at a high pressure in the order of 155 bars by means of a pressurizer 22 that is connected to one of the loops of the primary side.
The pressurizer makes it possible to keep the pressure in the primary circuit between predetermined limits either by spraying the primary coolant fluid when the pressure tends to exceed the permissible upper limit or by electrical heating of the primary fluid when the pressure tends to fall below the permissible lower limit. These operations are carried out inside the pressurizer which comprises a generally cylindrical casing arranged with its axis vertical and having its lower and upper parts closed by means of domed ends. The lower domed end has sleeves passing through it in which electrical heaters are introduced into the pressurizer. The lower domed end also has a combined inlet and outlet surge nozzle that communicates directly with the primary loop piping 20 to maintain the pressure within the primary circuit within design limits.
As can be appreciated from FIGS. 2, 3 and 5, the surge nozzles 24 of the pressurizers 22 include thermal sleeves or liners 26 to reduce the effect of thermal transients on the fatigue of the nozzles. These thermal sleeves have typically been welded to or explosively expanded into the nozzle 24. FIG. 2 shows the thermal sleeve 26 welded at one axial location 28 along the interior of the nozzle. A spacer 29 is positioned between the thermal sleeve 26 and the nozzle 24, proximate an inner end to minimize vibration of the sleeve, to keep the sleeve centered in the nozzle during welding, and to maintain a radial gap between the nozzle and the sleeve as a thermal barrier. FIG. 3 shows the thermal sleeve 26 explosively expanded at the expansion zone 30, into the interior surface of the surge nozzle 24. Both of these installation techniques have drawbacks. Welding the thermal sleeve to the nozzle occurs only over a portion of the circumference, since welding over the entire circumference would result in unacceptable stresses in the thermal sleeve during certain transients. This results in non-uniform by-pass behind the thermal sleeve and bending in the nozzle. More particularly, the welding occurs on the interior of the nozzle typically over a 45° arc length. During cold water in-surge transients, the thermal sleeve contracts relative to the nozzle, and the asymmetric welding pattern results in a gap between the thermal sleeve and nozzle opposite the weld. Explosive expansion can also result in non-uniform expansion, and residual stresses in the sleeve material. The thermal sleeve is tightly fit to a groove machined into the cladding. There is no feature to center the thermal sleeve in the nozzle so contraction of the thermal sleeve during cold in-surge transients will result in non-uniform radial gaps, and hence additional thermal and bending stresses in the nozzle. In addition, explosive expansion is not always a well controlled process, and requires special permitting and handling which creates difficulties for the manufacturers.
Accordingly, an improved means for attaching the thermal sleeve to the nozzle is desired that will keep the thermal sleeve centered in the nozzle and not create non-uniform gaps between the sleeve and the interior of the nozzle.